The circadian clock is an intrinsic time-keeping mechanism that controls the daily rhythms of many physiological processes, such as sleep/wake behavior, body temperature, hormone secretion, and metabolism (Takahashi, J. S. et al. Nat. Rev. Genet. 2008, 9, 764; Green, C. B. et al. Cell, 2008, 134, 728; Zhang, E. E. et al. Nat. Rev. Mol. Cell. Biol. 2010, 11, 764). Circadian rhythms are generated in a cell-autonomous manner through transcriptional regulatory networks of clock genes. In the core feedback loop, the transcription factors CLOCK and BMAL1 activate expression of Period (Per1 and Per2) and Cryptochrome (Cry1 and Cry2) genes. After translation and nuclear localization, PER and CRY proteins inhibit the function of CLOCK-BMAL1, resulting in sustained rhythmic gene expression. Many physiological pathways are under the control of the circadian clock (Panda, S. et al. Cell, 2002, 109, 307), including direct regulation of numerous hepatic processes (Rey, G. et al. PLoS Biol. 2011, 9, e1000595; Bugge, A. et al. Genes Dev. 2012, 26, 657).
Circadian desynchrony has been associated with impaired insulin sensitivity (Spiegel, K. et al. J. Appl. Physiol. 2005, 99, 2008; Spiegel, K. et al. Lancet, 1999, 354, 1435), decreased leptin levels and results in hyperglycemia, hyperinsulinemia and postprandial glucose responses comparable to those of a prediabetic state (Scheer, F. A. et al. Proc. Natl. Acad. Sci. USA, 2009, 106, 4453). Several genome-wide association studies led to the discovery that Cry2 may be important in the regulation of mammalian glucose levels (Dupuis, J. et al. Nat. Genet. 2010, 42, 105; Liu, C. et al. PLoS One, 2011, 6, e21464; Barker, A. et al. Diabetes, 2011, 60, 1805).
Glucose concentrations in the blood are highly rhythmic because of changes in insulin sensitivity and insulin secretory capacity of the endocrine pancreas (Polonsky, K. S. et al. N. Engl. J. Med. 1988, 318, 1231). ClockΔ19 mutant mice develop age-dependent hyperglycemia and these animals also develop susceptibility to diet-induced obesity, have inappropriately low concentrations of insulin (Turek, F. W. et al. Science, 2005, 308, 1043) and display a steeper drop in blood sugar in response to treatment with insulin, indicating that these animals have enhanced insulin sensitivity, thereby masking their β-cell deficiency (Marcheva, B. et al. Nature, 2010, 466, 627). Liver-specific deletion of Bmal1 in mice results in impaired glucose tolerance and increased insulin sensitivity (Lamia, K. A. et al. Proc. Natl. Acad. Sci. USA, 2008, 105, 15172). Individuals with type 2 diabetes, and even their first-degree relatives not yet affected with the disease, display altered rhythmicity in glucose tolerance (Boden, G. et al. Diabetes, 1999, 48, 2182). Also, Per2, Per3, and Cry2 expression is significantly lower in humans with type 2 diabetes versus humans without the disease (Stamenkovich, J. A. et al. Metabolism, 2012, 61, 978). The gluconeogenic genes phosphoenol pyruvate carboxykinase (Pckl) and glucose 6-phosphatase (G6pc) are controlled by CRY and the Bmal1 gene regulator REV-ERB (Zhang, E. E. et al. Nat. Med. 2010, 16, 1152; Lamia, K. A. et al. Nature, 2011, 480, 552; Yin, L. et al. Science, 2007, 318, 1786). Gluconeogenesis is tightly controlled by multiple signaling mechanisms and moreover, studies in mice have revealed that modulation of Cry1 and Cry2 can perturb gluconeogenesis and regulate blood sugar levels (Zhang, E. E. et al. Nat. Med. 2010, 16, 1152).
In a monotherapeutic or combination therapy context, new and established oral antidiabetic agents have non-uniform and limited effectiveness. Oral antidiabetic therapies suffer from poor or limited glycemic control, or poor patient compliance due to unacceptable side effects, such as edema, weight gain, or even more serious complications like hypoglycemia. Metformin, a substituted biguanide, can cause diarrhea and gastrointestinal discomfort. Finally, edema, weight gain, and in some cases, hepatotoxicity and cardiotoxicity, have been linked to the administration of some thiazolidine-2,4-dione antidiabetic agents (e.g. Rosiglitazone and Pioglitazone). Combination therapy using two or more of the above agents is common, but generally only leads to incremental improvements in glycemic control.
Cry1 and Cry2 also interact with the glucocorticoid receptor (GR) to globally alter the transcriptional response to glucocorticoids (Lamia, K. A. et al. Nature, 2011, 480, 552). Loss of Cry1 and/or Cry2 results in glucose intolerance and constitutively high levels of circulating corticosterone, suggesting reduced suppression of the hypothalamic-pituitary-adrenal axis coupled with increased glucocorticoid transactivation in the liver. Genomically, Cry1 and Cry2 associate with a glucocorticoid response element in the Pckl promoter in a hormone-dependent manner, and dexamethasone-induced transcription of the Pckl gene was strikingly increased in cryptochrome-deficient livers. This suggests that the undesirable metabolic side effects of glucocorticoids (e.g. hyperglycemia, insulin resistance and suppression of adrenal function) used to suppress inflammation may be alleviated by combining them with agents that can stabilize Cry1 and/or Cry2.